The SETI League provides a set of dimensions to build a hydrogen horn. This has two components – the waveguide element at the far end which must have specific dimensions to tune it to neutral hydrogen. There is also the horn element at the front. It is designed to have dimension across horizontal access at front of around 950mm or so. Can this horn element be scaled to make it smaller or larger by any %? For example can I simply change dimensions by 70% and it still work?
Yes, but only within limits.
The SETI League’s set of dimensions to build a hydrogen horn can be found at https://setileague.org/articles/horn.htm
SETI League Horn of Plenty Dimensions:
Typical hydrogen rectangular waveguide, found at bottom of a hydrogen horn (although note the SETI League prototype built the waveguide into the bottom of the flared section). This is a professionally built one, but they can be made by hand or constructed from a 1 gallon (5 litre) rectangular paint/oil tin.
The waveguide section and the horn section serve different purposes:
The waveguide throat (including the probe position, backshort, and waveguide dimensions) is frequency-critical. Those dimensions are chosen so the dominant TE₁₀ mode propagates efficiently at 1420.40575 MHz. They should not be scaled if you want the horn to operate at the hydrogen line.
The flared horn is less critical. Its job is to transform the impedance from the waveguide to free space and determine the beamwidth and gain.
If you scaled only the horn to 70% of the published dimensions while keeping the waveguide unchanged:
It would still receive 1420 MHz.
The gain would decrease.
The beam would become wider.
The impedance match would usually become worse, so the return loss (VSWR) would increase somewhat.
Conversely, making the horn larger would generally:
Increase gain.
Narrow the beam.
Increase physical size and weight.
Eventually require the horn length and flare angle to be re-optimised.
As a rough guide:
Scaling by ±10–20% is usually acceptable with only a modest performance penalty.
70% of the original size is a substantial change. It will still function, but it is no longer the same horn design and you should expect noticeably lower efficiency and gain. It is better to redesign the horn than simply shrink every dimension.
For a horn intended specifically for hydrogen-line radio astronomy, I would generally avoid shrinking the aperture below about 80–85% of the original SETI League design unless there is a compelling reason.
If the goal is to make the antenna more portable or fit a particular space, below is an estimation of how much gain and beamwidth would lose by reducing the aperture from about **950 mm to 665 mm (70%)**, and whether that loss is acceptable for Galactic hydrogen observations.
Using the SETI League horn as a reference:
Original aperture: ~950 mm
Scaled aperture (70%): ~665 mm
At a wavelength of 21.1 cm (1420 MHz), the aperture is reduced from about 4.5 wavelengths across to 3.1 wavelengths.
Approximate performance would be:
Aperture Gain 3 dB Beamwidth
950 mm 15–16 dBi ~15°
665 mm (70%) 12–13 dBi ~22°
So reducing the aperture to 70% would typically result in:
About 3 dB less gain, meaning you collect roughly half the signal power from the hydrogen line.
A beam around 50% wider, which reduces angular resolution but allows you to see a larger area of sky at once.
For observing the Milky Way’s neutral hydrogen, a 12–13 dBi horn is still quite usable if paired with:
a low-noise amplifier close to the feed,
a good 1420 MHz filter, and
sufficient integration time.
If portability is your main concern, a 665 mm horn is a reasonable compromise.
If, however, your aim is to maximise sensitivity, I would recommend keeping the full 950 mm aperture or even increasing it to around 1.2 m, as the gain continues to improve.
Below are calculations for the expected gain for apertures of **600 mm, 750 mm, 950 mm, and 1200 mm** so can compare the trade-offs directly.
At the hydrogen line (1420.40575 MHz, wavelength ≈ 211 mm), the horn’s gain is approximately proportional to its aperture area, so doubling the area increases the gain by about 3 dB.
Here’s a comparison, assuming a horn with similar proportions to the SETI League design:
Front aperture Aperture (wavelengths) Approx. Gain Approx. 3 dB Beamwidth
600 mm 2.8 λ 11–12 dBi 24–26°
665 mm 3.1 λ 12–13 dBi 21–23°
750 mm 3.6 λ 13–14 dBi 18–20°
950 mm 4.5 λ 15–16 dBi 14–16°
1200 mm 5.7 λ 17–18 dBi 11–12°
What this means in practice
600 mm: Compact and portable. Suitable for detecting the Galactic hydrogen line with a good LNA and enough integration time.
665 mm (70%): A good compromise between size and performance.
750 mm: Noticeably better sensitivity while still significantly smaller than the original.
950 mm: The SETI League design offers an excellent balance of gain, beamwidth, and manageable size.
1200 mm: Higher sensitivity and finer angular resolution, but substantially larger and heavier.
A useful design guideline
Rather than scaling every dimension by the same factor, many microwave engineers would:
Keep the waveguide section exactly as designed (probe, backshort, and throat dimensions unchanged).
Keep the flare angle the same.
Choose whatever final aperture size you want.
Adjust only the horn length so the flare angle remains constant.
This preserves much of the horn’s performance better than simply shrinking every dimension, because the waveguide transition remains optimised for 1420 MHz.
Below is an attempt to derive a 750 mm or 665 mm version of the SETI League horn that keeps the waveguide section unchanged and recalculates the horn dimensions for the chosen aperture. That approach should perform better than a simple 70% scale-down.
Thank you. These are sufficient to work from.
The original SETI League horn has:
Length: 48 in (1219 mm)
Front aperture: 36 × 27 in (914 × 686 mm)
Throat: 6.5 × 3.25 in (165 × 82.5 mm) – this mates to the waveguide and should not be changed.
A redesigned horn with a 70% aperture
Rather than scaling every dimension to 70%, I recommend keeping:
Throat: 165 × 82.5 mm (unchanged)
Horn length: adjusted to preserve the flare angle.
This gives:
Original New
Aperture width 914 mm 640 mm
Aperture height 686 mm 480 mm
Horn length 1219 mm ≈840–860 mm
Notice the horn becomes considerably shorter than the original because the flare angle is maintained.
If you simply scaled everything by 70%
You would obtain:
Length = 853 mm
Aperture = 640 × 480 mm
Throat = 116 × 58 mm ❌
The throat would no longer match the hydrogen-line waveguide, causing poor impedance matching and reduced efficiency.
Possible recommendation.
Use:
Waveguide section: exactly as published by SETI League.
Horn throat: 165 × 82.5 mm.
Horn length: about 850 mm.
Front opening: 640 × 480 mm.
This should give approximately:
Gain: 12.5–13 dBi
Beamwidth: 22°
Good performance on the 21 cm hydrogen line.
Another possibility
Since you’ve previously mentioned wanting a more portable hydrogen-line telescope, I’d actually suggest a 750 mm wide version rather than 640 mm.
A 750 × 560 mm aperture with a horn length of about 1.0 m would still be much easier to transport than the original but would recover roughly 1 dB of gain compared with the 640 mm version. That may not sound like much, but in radio astronomy it is worthwhile.. This has two components – the waveguide element at the far end which must have specific dimensions to tune it to neutral hydrogen. There is also the horn element at the front. It is designed to have dimension across horizontal access at front of around 950mm or so. Can this horn element be scaled to make it smaller or larger by any %? For example can I simply change dimensions by 70% and it still work?
Yes, but only within limits.
The waveguide section and the horn section serve different purposes:
The waveguide throat (including the probe position, backshort, and waveguide dimensions) is frequency-critical. Those dimensions are chosen so the dominant TE₁₀ mode propagates efficiently at 1420.40575 MHz. They should not be scaled if you want the horn to operate at the hydrogen line.
The flared horn is less critical. Its job is to transform the impedance from the waveguide to free space and determine the beamwidth and gain.
If you scaled only the horn to 70% of the published dimensions while keeping the waveguide unchanged:
It would still receive 1420 MHz.
The gain would decrease.
The beam would become wider.
The impedance match would usually become worse, so the return loss (VSWR) would increase somewhat.
Conversely, making the horn larger would generally:
Increase gain.
Narrow the beam.
Increase physical size and weight.
Eventually require the horn length and flare angle to be re-optimised.
As a rough guide:
Scaling by ±10–20% is usually acceptable with only a modest performance penalty.
70% of the original size is a substantial change. It will still function, but it is no longer the same horn design and you should expect noticeably lower efficiency and gain. It is better to redesign the horn than simply shrink every dimension.
For a horn intended specifically for hydrogen-line radio astronomy, I would generally avoid shrinking the aperture below about 80–85% of the original SETI League design unless there is a compelling reason.
If the goal is to make the antenna more portable or fit a particular space, below is an estimation of how much gain and beamwidth would lose by reducing the aperture from about **950 mm to 665 mm (70%)**, and whether that loss is acceptable for Galactic hydrogen observations.
Using the SETI League horn as a reference:
Original aperture: ~950 mm
Scaled aperture (70%): ~665 mm
At a wavelength of 21.1 cm (1420 MHz), the aperture is reduced from about 4.5 wavelengths across to 3.1 wavelengths.
Approximate performance would be:
Aperture Gain 3 dB Beamwidth
950 mm 15–16 dBi ~15°
665 mm (70%) 12–13 dBi ~22°
So reducing the aperture to 70% would typically result in:
About 3 dB less gain, meaning you collect roughly half the signal power from the hydrogen line.
A beam around 50% wider, which reduces angular resolution but allows you to see a larger area of sky at once.
For observing the Milky Way’s neutral hydrogen, a 12–13 dBi horn is still quite usable if paired with:
a low-noise amplifier close to the feed,
a good 1420 MHz filter, and
sufficient integration time.
If portability is your main concern, a 665 mm horn is a reasonable compromise.
If, however, your aim is to maximise sensitivity, I would recommend keeping the full 950 mm aperture or even increasing it to around 1.2 m, as the gain continues to improve.
Below are calculations for the expected gain for apertures of **600 mm, 750 mm, 950 mm, and 1200 mm** so can compare the trade-offs directly.
At the hydrogen line (1420.40575 MHz, wavelength ≈ 211 mm), the horn’s gain is approximately proportional to its aperture area, so doubling the area increases the gain by about 3 dB.
Here’s a comparison, assuming a horn with similar proportions to the SETI League design:
Front aperture Aperture (wavelengths) Approx. Gain Approx. 3 dB Beamwidth
600 mm 2.8 λ 11–12 dBi 24–26°
665 mm 3.1 λ 12–13 dBi 21–23°
750 mm 3.6 λ 13–14 dBi 18–20°
950 mm 4.5 λ 15–16 dBi 14–16°
1200 mm 5.7 λ 17–18 dBi 11–12°
What this means in practice
600 mm: Compact and portable. Suitable for detecting the Galactic hydrogen line with a good LNA and enough integration time.
665 mm (70%): A good compromise between size and performance.
750 mm: Noticeably better sensitivity while still significantly smaller than the original.
950 mm: The SETI League design offers an excellent balance of gain, beamwidth, and manageable size.
1200 mm: Higher sensitivity and finer angular resolution, but substantially larger and heavier.
A useful design guideline
Rather than scaling every dimension by the same factor, many microwave engineers would:
Keep the waveguide section exactly as designed (probe, backshort, and throat dimensions unchanged).
Keep the flare angle the same.
Choose whatever final aperture size you want.
Adjust only the horn length so the flare angle remains constant.
This preserves much of the horn’s performance better than simply shrinking every dimension, because the waveguide transition remains optimised for 1420 MHz.
Below is an attempt to derive a 750 mm or 665 mm version of the SETI League horn that keeps the waveguide section unchanged and recalculates the horn dimensions for the chosen aperture. That approach should perform better than a simple 70% scale-down.
Thank you. These are sufficient to work from.
The original SETI League horn has:
Length: 48 in (1219 mm)
Front aperture: 36 × 27 in (914 × 686 mm)
Throat: 6.5 × 3.25 in (165 × 82.5 mm) – this mates to the waveguide and should not be changed.
A redesigned horn with a 70% aperture
Rather than scaling every dimension to 70%, I recommend keeping:
Throat: 165 × 82.5 mm (unchanged)
Horn length: adjusted to preserve the flare angle.
This gives:
Original New
Aperture width 914 mm 640 mm
Aperture height 686 mm 480 mm
Horn length 1219 mm ≈840–860 mm
Notice the horn becomes considerably shorter than the original because the flare angle is maintained.
If you simply scaled everything by 70%
You would obtain:
Length = 853 mm
Aperture = 640 × 480 mm
Throat = 116 × 58 mm ❌
The throat would no longer match the hydrogen-line waveguide, causing poor impedance matching and reduced efficiency.
Possible recommendation.
Use:
Waveguide section: exactly as published by SETI League.
Horn throat: 165 × 82.5 mm.
Horn length: about 850 mm.
Front opening: 640 × 480 mm.
This should give approximately:
Gain: 12.5–13 dBi
Beamwidth: 22°
Good performance on the 21 cm hydrogen line.
Another possibility
Since you’ve previously mentioned wanting a more portable hydrogen-line telescope, I’d actually suggest a 750 mm wide version rather than 640 mm.
A 750 × 560 mm aperture with a horn length of about 1.0 m would still be much easier to transport than the original but would recover roughly 1 dB of gain compared with the 640 mm version. That may not sound like much, but in radio astronomy it is worthwhile.